Bag house filtration systems are primarily used in industrial environments to remove dust, and contaminants and other particles from an air stream. The purpose of bag house filtration systems can be pollution control to improve the air quality in a confined environment, or it can be to reclaim particles found in air that can be reused in a manufacturing process e.g. cement factories, textile mills, carbon black production, furniture and woodworking plants or both. Bag house filtration systems circulate air through the wall(s) of a bag which collects or reclaims the particles. The bag, typically formed from a textile fabric or media, performs as both a barrier to the particles and a conduit for air to pass through. The dual functionality required for modern filter media have focused on three objectives: effectively remove particles from the air, reduce the energy required to circulate air, and to enhance the cleaning of the bags as needed.
The ability to clean the air stream circulated through the bag house systems has been referred to in the art as filtration efficiency. A measure of filtration efficiency is the ratio of downstream (output) particle concentration compared to the upstream (input) particle concentration that has passed through the bag or filter media. Particle size and air flow rates impact filtration efficiency, however, in general the higher the aforesaid ratio the more efficient the bag filter media. As more particulate is collected on the surface of the filter media the energy required to transport air through the bag (or any filter media) also increases, thus the balance between effective air cleaning and energy usage is delicate.
Energy usage in a bag house system is related to the pressure drop through the bags or filter media. Pressure drop, as used herein, refers to the difference in the rate of air flow between each side of a filter media. A low pressure drop media requires less energy to push air through the bag lessening the overall energy consumption of the system. Over time a phenomena referred to as filter cake formation occurs where the bag collects particles on its surface. As the filter cake increases the pressure drop of the filtration system increases the energy requirements of the bag house filtration system. Thus, the bag surfaces should be kept relatively clean in order that the filter operation can continue without interruption and with a minimum of energy usage.
Various approaches are used to control filter cake formation and remove the filter cake once formed. A method typically used in reclamation type bag house systems includes mechanically shaking the bag so that the filter cake will flake off. Another approach utilizes a pulse of air abruptly flexing the filter media. The latter method is used most in pollution control or abatement systems. Both methods are very dynamic applications resulting in wear of the bag. Thus, the less often the cleaning process occurs and the easier the particles are to remove, the better the bag life.
In addition, due to the desire for increased filtration efficiency (trapping of more particles and smaller particles), advances have been made in which porous substrates have been used as supporting materials for a porous polymeric film or membrane which actually forms the filtration media. In such bag house filtration systems the substrate is generally formed from a woven textile material, which will have laminated thereto a polymeric film as discussed above.
In some cases for bag house usage, there have been developed filter media having conductive features and capable of generating an electric charge. However, the conductivity and electric charge of such filter media is for the purpose of attracting and fixing particles on the surfaces filter media. This will enhance filtration efficiency. Such materials have been used in cartridge and precipitator type filtration systems. Thus, these systems serve to improve the particle attraction and fixation onto the surface of a given filter media by charging the conductive particles so that the particles to be collected are attracted to the filtration media itself.